Compounds which contain the characteristic group: ##STR2## and which will hereinafter be referred to as "oxazolidines" for the sake of simplicity, have the interesting property that by the action of water (moisture) they may be converted hydrolytically into derivatives which contain hydroxyl and secondary amino groups, i.e., H0-R.sub.1 -NH groups. These oxazolidines are therefore potential reactants for organic polyisocyanates. Mixtures of such oxazolidines with polyisocyanates are therefore systems which may be hardened by the action of water. According to the teaching of German Offenlegungsschrift No. 2,018,233 and U.S. Pat. No. 3,743,626, this principle is applied to certain polyester oxazolidines which may be prepared in accordance with German Offenlegungsschrift Nos. 1,952,091 and 1,952,092 and U.S. Pat. Nos. 3,661,923 and 3,864,335. The polyester oxazolidines disclosed in the said publications, however, have certain disadvantages with regard both to the process by which they are produced and to the properties of the materials produced from them in accordance with this principle. They are prepared by a transesterification reaction which proceeds slowly even in the presence of catalysts and which must be carried out at temperatures below 160.degree. C. in order to prevent side reactions. The ester groups are found unchanged in the end-products obtained after hydrolytic ring opening followed by the isocyanate polyaddition reaction, so that even the completely hardened end-products of German Offenlegungsschrift No. 2,018,233 have the major disadvantage common to all synthetic resins which contain ester groups of having little resistance to hydrolysis.